In the past, rotary magnetic encoders employed an external application magnet attached to an end of a rotary encoder shaft rotatively supported by one or more shaft bearings that were required to align the rotating shaft to accurately locate the magnet relative to Hall sensors of a magnetic field sensor of an onboard rotary magnetic encoder chip. Shaft bearings were required to maintain precise shaft alignment to locate the external magnet relative to the Hall sensors because the Hall sensors require a uniform magnetic field distribution to be provided by the magnet. Failure to provide a desirably uniform magnetic field distribution, such as what can happen if shaft misalignment due to bearing wear occurs, can cause undesirably high signal noise, positional error, or both.
Commonly owned U.S. application Ser. No. 12/205,902 discloses a shaft misalignment tolerant rotary magnetic encoder having an internal magnet received in a pocket formed in the body of a low magnetic permeability encoder housing located adjacent the rotary magnetic encoder chip that is rotatively driven by an external drive magnet mounted to the encoder shaft. During operation, rotation of the encoder shaft rotates the external drive magnet substantially in unison therewith magnetically coupling the external magnet to the internal magnet within the encoder housing causing the internal magnet to rotate substantially in unison with the external drive magnet. As a result of being located in close proximity to the Hall sensors of the encoder chip, the internal magnet provides a desirably uniform magnetic field distribution even when shaft misalignment occurs. Such a rotary magnetic encoder of shaft misalignment tolerant construction can be of more economical construction as it does not require shaft bearings adjacent the encoder housing to rotatively support and align the encoder shaft.
While noncontact or “contactless” shaft misalignment tolerant rotary magnetic encoders have enjoyed considerable commercial success, improvements nonetheless remain desirable. For example, as the internal magnet spins within encoder housing, it can come into contact with part of the housing causing the angular velocity of the internal magnet to non-uniformly vary, which can cause an undesirable magnitude of angle error and a hysteresis phenomenon known as “Quiver.” During encoder operation, the rotating internal magnet, particularly when rotating at relatively high speeds can levitate or lift causing it to “float” within the encoder housing inducing drag on the internal magnet which can also cause an undesirable magnitude of angle error.
What is needed is a shaft misalignment tolerant rotary magnetic encoder of improved construction that minimizes angle error during encoder operation.